Polyimide resin is widely used for electric and electronic purposes because the polyimide resin has an excellent heat resistance, electrical insulation reliability, chemical resistance, and mechanical property. For example, the polyimide resin is used for forming (A) insulating films and protective coating agents onto semiconductor devices, (B) base materials and surface protective materials of flexible circuit substrates, integrated circuits, etc., and (C) interlayer insulating films and protective films of extremely small circuits.
There has been used a cover lay film particularly as a surface protective material for flexible circuit substrates, which cover lay film can be obtained by applying an adhesive to a molding product such as a polyimide film. The cover lay film is adhered to a flexible circuit substrate generally in such a way that an opening is secured by means of punching etc. in advance at a bonding portion between the cover lay film and a terminal portion or component of a circuit, the opening and the cover lay film are aligned with each other, and then the cover lay film and the flexible circuit substrate are subjected to thermocompression bonding with use of a thermal press etc.
It is, however, difficult to secure an opening in a thin cover lay film with a high accuracy, and the alignment to bond the cover lay film and the flexible circuit substrate to each other is usually carried out manually. This causes insufficiently accurate alignment, low workability at the time of bonding, and high costs.
Meanwhile, a solder resist or the like is also used as a surface protective material for circuit substrates, and, particularly, a solder resist having a photosensitivity function is preferably used in a case where a fine processing is necessary. Such a photosensitive solder resist is made from a photosensitive resin composition mainly containing acid-modified epoxy acrylate, epoxy resin, etc. The photosensitive solder resist has an excellent electrical insulation reliability as an insulating material. However, the photosensitive solder resist has a bad mechanical property such as flexibility, and is largely shrunk when being cured. Therefore, a warpage of a circuit substrate becomes large in a case where the photosensitive solder resist is laminated onto a thin and plastic circuit substrate such as a flexible circuit substrate. This makes it difficult to use a photosensitive solder resist for flexible circuit substrates. The photosensitive solder resist has poor flame retardancy, and therefore, in a case where a flame retardant is added to the photosensitive solder resist to apply flame retardancy, the photosensitive solder resist causes problems such as (i) reduction in physical property and (ii) a contact fault and contamination during processes, each of which is caused by bleed out which is a phenomenon that a flame retardant is soaked out of a cured film.
There have been proposed various photosensitive solder resists which can exhibit flexibility and flame retardancy.
For example, Patent Literature 1 discloses a photosensitive resin composition which (i) has excellent flexibility, excellent solder heat resistance, high sensitivity, and high resolution, and (ii) can makes it easy to form a fine pattern on a heat-resistant protective film.
One of important properties in a case where a photosensitive solder resist is processed is a tack-free property, i.e., low tackiness of a surface after the surface is applied with a material of the film and a solvent is dried. The high tack-free property is an important property because (i) the high tack-free property prevents a photomask, which is necessary for forming a fine pattern, from adhering to a surface of an applied film to thereby contaminate the surface in a case where the photomask is provided on the surface of the applied film and the surface is radiated by ultraviolet rays, and (ii) the high tack-free property prevents circuit substrates from adhering to each other in a case where the circuit substrates applied with films which have not been radiated yet with ultraviolet rays are stacked on one another.
As a method for improving the tack-free property after the photosensitive solder resist is applied and dried, there employs a method of adding inorganic fillers to cause an applied film to have a rough surface. The method, however, has a problem in that the film after curing (cured film) becomes weak because the inorganic fillers are hard, and cracking occurs on the cured film, thereby causing peeling-off of the cured film from a base material.
In view of the circumstances, for example, Patent Literature 2 discloses a high performance ink composition for use in a photo-curable liquid solder resist, which ink composition (A) has a high sensitivity and excellent tack-free and developing properties of a film and (B) does not decrease adhesiveness or does not peel off due to cracking or/and volumetric shrinkage.